An Australian man who bought a baby boy for $US8000 (NZ$10,321) with his partner, sexually abused the child and handed him off to other pedophiles to molest, has been sentenced to 40 years' prison in the US.

Judge Sarah Evans Barker, while sentencing the 42-year-old in the US District Court in Indianapolis on Friday, said he deserved a harsher punishment but accepted the plea deal because she did not want to subject jurors to the disturbing evidence.

Prosecutors discovered videos and photos of the man, his domestic partner, also an Australian, and other men in Australia, the US, Germany and France abusing the child from the age of two to six.

''For more than one year and across three continents, these men submitted this young child to some of the most heinous acts of exploitation that this office has ever seen,'' Indiana US Attorney Joe Hogsett said after the sentencing.

The 40-year sentence was the maximum available for the conspiracy to sexually exploit a minor and conspiracy to posses child pornography charges the man entered guilty pleas to.

His partner has also entered guilty pleas and will be sentenced later this year.

The court heard the two men, who had been living in California, bought the newborn boy in an undisclosed country and used falsified documents in Los Angeles in an attempt to adopt him.

The boy was rescued by US authorities and is being cared for in California.